Lenz’s Law

Lenz’s Law states that the induced e.m.f. will be directed such that the current which it causes to flow opposes the change that is producing it.

  • The induced e.m.f. is always present even when the conductor is not in a complete circuit. BUT there will only be an induced current flowing only if the set-up is in a complete circuit.

A bar magnet with the North pole closer to a coil of wires is moved towards the coil. The current induced will be such that a North pole is created on the side of the coil that the magnet is approaching. This “created” north pole repels the approaching N pole of the magnet.

If such a magnet as above is moved away from the coil, the current induced in the coil will be such that a South pole is created on the side the North pole of the magnet is leaving. This “created” south pole attracts the North pole of the magnet.

The negative sign in the equation $\epsilon = \, – \, \frac{d \Phi}{dt}$ indicates that the induced e.m.f. and the change in flux have opposite signs. The polarity of the induced e.m.f. is such that it tends to produce a current that creates a magnetic flux to oppose the change in magnetic flux through the area enclosed by the loop.

Lenz’s law in action:



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